Semipermeable membranes, in ultrafiltration and reverse osmosis processes, have been used in the past for the separation or concentration of a variety of substances including those which can support the growth of microorganisms.
Osmosis is a process which can operate by confining or restraining a fluid with a semipermeable membrane designed to permit the passage through the membrane of certain fluid constituents while preventing the passage of other constituents. A semipermeable membrane separating two solutions of differing concentrations or solvent activities permits the solvent to flow from the more dilute solution through the membrane into the more concentrated solution allowing the concentration on either side of the membrane to equilibrate.
In reverse osmosis, pressure is deliberately applied to the more concentrated solution causing the flow of solvent in the opposite direction through the membrane, i.e. into the more dilute solution. In this way the diluent liquid can be separated from dissolved species, increasing the concentration of the species in solution. Reverse osmosis is ideal for sensitive biochemical and organic compositions since it relies on pressure rather than heat or other energetic processes. Reverse osmosis also has the substantial benefit, in comparison to other water separation processes such as distillation and freeze drying, of being a process having low energy consumption.
Ultrafiltration is a filtration process for the removal of particles that are unfilterably small for conventional filtration processes.
Semipermeable membranes provide an ideal environment for the growth of a variety of microorganisms in and on the surfaces of the membrane when contacted with a solution containing nutrient components. The membranes are flawed by small microscopic pores which can cause some minor leaks and some small amount of mixing between the solutions on either side of the membrane. Accordingly, while one side of the membrane is exposed to the high concentrations of the dissolved, commonly nutrient species, a small concentration of the dissolved nutrient species can appear on the reverse of the membrane.
A colony of microorganisms can begin to grow around a small particle of a nutrient substance from the solution, on either side of the membrane. As the microorganisms grow and divide, supported by nutrients in the fluid, the colony can spread across the surface. The rate of flow of fluid through the membrane can be reduced in direct relationship to the surface area covered by or lost to the microorganism colonies. Since the efficiency of the reverse osmosis or ultrafiltration process is directly related to the flow rate of liquid through the membrane any reduction in fluid flow rate is highly undesirable. Further microorganism growth can spoil the fluid and constituent being separated or concentrated using the membrane.
Microorganism colonies can be removed from semipermeable membranes by mechanically or abrasively removing the microorganisms or by back flushing the reverse osmosis systems. However, these methods are inappropriate since semipermeable membranes are delicate, can be damaged by reverse flushing and can be destroyed by mechanical abrasion.
Chemical means for preventing or destroying microorganism growth have been used by contacting chlorine, formaldehyde hydrogen peroxide, etc. with the membranes. These chemicals can be harmful to operating personnel and can damage both the semipermeable membrane and substances and diluents in contact with the membrane.
Since the microbial growth can occur on either side or within the membrane, in order to be effective in killing microbs in the units when only one side of the membrane is available to treatment the antimicrobial composition must be able to pass through the membrane. Most common antimicrobial compounds comprising organic molecules having molecular weight greater than about 150 cannot successfully prevent or destroy microbial growth within membranes and on both surfaces since they cannot pass through the membrane.
A need exists for an effective, safe membrane-compatible composition which can be used to prevent or destroy microorganism growth in and on semipermeable membrane surfaces.